Issue1129b.agda:22,3-24,20
Don't know how to parse act (a , b) ← mAB , return $ b , a. Could
mean any one of:
  id (bind mAB λ _ @ (a , b) → (((return $) b) , a))
  id (bind (mAB , ((return $) b)) λ _ @ (a , b) → a)
Operators used in the grammar:
  ,    (infix operator, level 20) [_,_ (Issue1129b.agda:3,3-6)]  
  _←_, (prefix notation, level 1) [bind (Issue1129b.agda:10,1-5)]
  act  (prefix notation, level 0) [id (Issue1129b.agda:14,1-3)]  
when scope checking act (a , b) ← mAB , return $ b , a
